Perry's campaign said he plans to focus his efforts on traditional retail campaigning in the closing weeks. His campaign announced plans for a 44-town bus tour starting next week.

Bachmann, who earlier this week had expressed reservations about Trump's impartiality, officially RSVPd no Thursday.

Conservative media group Newsmax announced last week it had partnered with Ion Television to host a debate, and had chosen Trump as the moderator.

Before the Perry and Bachmann announcements, Trump acknowledged that his role had given candidates pause.

"People are afraid that I'm going to run as an independent candidate and some of the people that are supposed to be in the debate have expressed that. And I don't want to give up that option, because it certainly is an option," he told CNN's Wolf Blitzer.

But he said he was surprised that Romney backed out. And as for Perry and others: "What do they have to lose?"

Jon Huntsman became the first Republican candidate to snub a planned GOP candidates' debate moderated by Donald Trump, with his campaign saying Republicans deserve "a serious discussion of the issues."

There will be no boardroom, and no one will get fired. But those who say the Republican primary race has been something of a reality show have new evidence in the announcement that Donald Trump has signed up to host a debate planned in Iowa, just a week before the leadoff caucuses.

Amid heightened speculation about whether he will seek the Democratic presidential nomination, Vice President Joe Biden remains at least a month from a decision and has largely left preparatory work to a small circle of trusted longtime aides.

Perched in his wheelchair, "Big Jim" Blake confidently rolls the wooden floors of his old shoot-'em-up saloon, founded here in 1893. The history of the Cowboy Bar is populated with drifters, outlaws and outliers, and its present proprietor brims with wild yarns and tall tales.